replace accidental unwraps with if let's

this provides not only some future compatibility with MSC4051,
but it just makes sense to not crash/error if we can't get a server_name
from the room ID and should just use the server_name from the sender
user's invite event. there is already code ahead that accounts for
an empty vector so this is safe.

Signed-off-by: strawberry <strawberry@puppygock.gay>